Power-combined amplifiers use a variety of technologies, including vacuum tube amplifiers such as traveling wave tubes, and solid-state amplifiers realized in GaAs, GaN, InP, SiGe, and silicon CMOS processes. There is a broad need for the ability to sense and correct transmission phases in combined amplifiers, because normal phase variations in unit amplifiers can degrade overall efficiency as some portion of the signal that should coherently combine at the power amplifier output is dissipated in isolation resistors. (Conservation of energy demands that dissipation in isolation resistors is subtracted from the potential output power of the amplifier, thus degrading overall efficiency.)
“Phase efficiency” in a power-combined amplifier is approximately related to RMS phase error between constituent amplifiers as:ηphase=cos2(φRMS).  (1)
FIG. 2 shows the effect on efficiency when the RMS phase error between amplifiers is considered in an N-way combiner following Eq. 1. In the case of a two-way divider, RMS error is equal to one-half the full error between the two signals (if they are 90 degrees out of phase, RMS error is 45 degrees). For example, if two amplifiers were combined that are 40 degrees apart in phase (20 degrees RMS), the phase efficiency would be 88.3% according to Eqn. 3. This means that without considering combiner real loss, combining two power amplifiers that have 40% power-added efficiency (PAE) but are 40 degrees apart, would lead to an output signal that is reduced to 88.3% of its ideal value (power is reduced by −0.54 dB) and 40% PAE amplifiers would be reduced to 35.3%. The missing power is converted to heat in the combiner's isolation resistors, which must be properly sized to handle the expected worst-case wasted power for reliable operation. Clearly it is undesirable to lose any efficiency points in this manner. A solution might be to line up all amplifiers in an N-way combiner to fit within an 11.5-degree window, ensuring that RMS effort is less than 5.7 degrees. Under this condition, 1% would be the maximum performance lost and phase efficiency would exceed 99%.
For reference, RMS error is defined as:
                                          φ            RMS                    =                                                    ∑                1                N                            ⁢                                                          ⁢                                                                    (                                                                  φ                        i                                            -                                              φ                        _                                                              )                                    2                                N                                                    ,                            (        2        )            where phase angles are typically expressed in degrees.
Prior attempts to solve the phase mismatch problem in power-combined amplifiers include binning component amplifiers into transmission phase windows based on measurements. However, in solid-state power amplifiers, for example, often only small-signal data is known prior to assembling the amplifier, and small-signal phase may be different from large-signal phase. Furthermore, amplifiers can change over time and temperature. Thus, a solution provided at initial device fabrication may fail or unacceptably degrade later with amplifier change. Also, effects of interconnects must be considered, including wirebonds and non-ideal phase performance of divider and combiner. Enforcing narrow phase bins on a widely varying but finite population of amplifier chips can force undesirable compromises in phase alignment, particularly during a rework event where an amplifier chip must be replaced using “leftover” amplifiers.
In contrast, phase trim has been used with vacuum-tube amplifiers to adjust phases of individual component amplifiers, in-situ, during large signal operation. The loss of the phase trim circuit is not important, as it is configured on the input side of the individual amplifiers. In such a case, the phase shifter is set to maximize the ratio of output power to wasted power, or minimize the wasted power for a fixed input power, through trial and error. A mechanical adjustment is used, and it is recommended that the adjustment be made at the factory. Further, the amplifier must be taken off-line to evaluate the phase mismatch.
In a similar manner, power sensing may be performed using isolation loads on hybrid couplers which include 90-degree hybrids and 180-degree hybrids; however, schemes such as these are not ideal and provide only one sense for every pair of amplifiers. One sense for two amplifiers can indicate the magnitude of the phase mismatch but not the direction (positive or negative) of the mismatch. Consequently using this type of power sensing for phase correction results in an iterative solution. In addition, hybrid couplers are generally not applicable to direct N-way combiners.
While it is theoretically possible to perform power sensing across the isolation resistor in a Wilkinson power divider, decoupling the DC signal from the RF at this most-sensitive node in the combiner is a tricky endeavor, which would result in reduced power combining efficiency due to common-mode loading. Schemes have been introduced that decouple the Wilkinson resistor to a one-port network using balun structures at the expense of bandwidth and complexity. Thus, it would be an advance in the art to overcome the above noted deficiencies in power-combined amplifiers.